the water levels hitting historic low levels and drying up a big part of the american economy. >>> and later, the healing power of a familiar face from home. pets now allowed, even encouraged at a growing number of hospitals. >>> the great lakes contain one fifth of the fresh water supply in the surface of the earth. that may seem like a great deal of it, these days it's not enough for the level of commerce that those five truly great lakes have supported for so long. the problem is the water levels. the lakes are currently low and getting lower. our report on the impact tonight from our chief environmental correspondent anne thompson. >> reporter: the cargo ship "mesabi miner" rides high as it docks on lake erie. and that's not good. >> she left 8,000 tons of iron ore in minnesota, that's basically a day's work for an iron ore mine. >> reporter: iron ore destined to be steel in cars and refrigerators. but the mesabi miner can't carry a full load because once again water levels in the great lakes are too low. >> it's reducing our revenue. we carry less tons, we get paid by the ton. we